ImplementStrStr.java

package easy._28;

/**
 * @author Larry
 *
 * Implement strStr().
 *
 * Return the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack.
 *
 * Example 1:
 *        Input: haystack = "hello", needle = "ll"
 *        Output: 2
 *
 * Example 2:
 *        Input: haystack = "aaaaa", needle = "bba"
 *        Output: -1
 *
 * Clarification:
 *
 * What should we return when needle is an empty string? This is a great question to ask during an interview.
 * For the purpose of this problem, we will return 0 when needle is an empty string. This is consistent to C's strstr() and Java's indexOf().
 */

public class ImplementStrStr {

    public int strStr(String haystack, String needle) {

        if (needle == null || needle.equals("")) {
            return 0;
        }

        for (int i = 0; i < haystack.length() - needle.length() + 1; i ++) {
            if (haystack.charAt(i) == needle.charAt(0)) {
                if (haystack.substring(i, i + needle.length()).equals(needle)) {
                    return i;
                }
            }
        }

        return -1;

    }

}